Generally, a vehicle compressor is applied to an air conditioning system so as to be driven by power received from an engine and functions to compress refrigerant having a lower pressure by an evaporator into high temperature and high pressure gaseous refrigerant and then transfer the high temperature and pressure gaseous refrigerant to a condenser.
The compressor receives the power through a pulley connected via a belt to a crankshaft. As shown in FIG. 1, the pulley includes a circular plate-shaped base portion 111 that is disposed perpendicularly to a length direction of a compressor 120, and internal ring portion 114 and external ring portion 116 which are respectively protruded from the base portion 111 in the length direction of the compressor 120. A belt (not shown) connected with a crankshaft of an engine is hanged on an outer surface of the external ring portion 116 so as to be rotated. Rotational force of the belt is transmitted to a shaft fixed to an end of an inner surface of the internal ring portion 114, and thus the compressor 120 performs a compressing process.
The pulley 110 having the above-mentioned function is typically made of a metallic material so as to be capable of enduring tensile force of the belt, and the metallic material for the pulley 110 is worked by a hot forging method which provides a lower strength compared to a cold forging method but a lower manufacturing cost.
FIG. 2 is a flow chart showing manufacturing processes of a conventional compressor pulley. In order to manufacture the pulley 110, first of all, the metallic material is prepared, cut into a desired size and then heated to a temperature of about 1,000˜1,250° C. through a heating process. The heated metallic material is formed into a pulley shape having the base portion 111, the internal ring portion 114 and the external ring portion 116 by a hot forging process. At this time, the metallic material is forged under a pressure of about 800˜1,300 tons applied by a press, and the forging process may be repeated one or more times according to the metallic material, the temperature, the expected shape of the pulley and the pressure of the press.
A center portion of the pulley 110 treated by the hot forging process is also pierced so as to form a through-hole 115 through which one end of the shaft is inserted in the internal ring portion 114, and then cooled to a room temperature by a cooling process. The cooled pulley 110 secures a dimension and a thickness for a slot piercing process while being treated in a rough grinding step. The pulley 110 treated in the rough grinding step has a slot 112 formed at the base portion 111 by the slot piercing process, and thus only a bridge portion 113 is remained. The pulley 110 treated by the slot piercing process is completed through a finishing step in which groove working and grinding are performed on a surface thereof.
The pulley 110 completed by the above-mentioned processes is connected with the shaft of the compressor 120 and rotates. As described above, since the pulley 110 is manufactured by the hot forging process in which a metallic material is hot-forged, it is facile to manufacture the pulley 110. However, since the strength of the bridge portion 113 on which a physical force is concentrated is insufficient, it is apprehended that the bridge portion 113 may be easily damaged.
To solve such the problem and improve the strength of the bridge portion 113, the cold forging process that the metallic material is forged at a room temperature is used to manufacture the pulley 110, instead of the hot forging process that the metallic material is hot forged, so as to improve the strength of the bridge portion 113. By the cold forging process, the strength of the bridge portion 113 is improved. However, since the strength of the other portions except the bridge portion 113 are also improved, it is difficult not only to perform the central piercing process for forming the through-hole 115 at the central portion of the base portion 111 and the slot piercing process for forming the slot 112 at the base portion 111, but also to form the groove portion 117 at the outer surface of the external ring portion 116, and also there are other problems that damage of a work tip used for forming the groove portion may be increased and it is difficult to ensure accuracy of the groove portion 17 upon semi-roll forming process. In addition, the hot forging process can be performed at a press pressure of about 800˜1,300 tons, but the cold forging process needs a press pressure of about 2,500 tons or more. Thus, there is another problem that a large-sized equipment for the cold forging process is required.
Further, as shown in FIG. 3, in the hot forging process that the heated metallic material is forged, a connecting portion 118 for connecting a center portion of the internal ring portion 114 is formed at a lower side of the internal ring portion 114 by a backward extruding process. Due to the connecting portion 118, serious inhomogeneous deformation and excessive barreling are occurred in a grain flow line formed around the bridge portion 113 comparing with a grain flow lines formed at other portions, and thus it becomes weak for stress and thus the strength thereof is lowered.